High pressure discharge lamps, for example high-pressure mercury discharge lamps, are currently used in diverse fields of application, for example in projectors.
It is important here to set the temperature of the lamp bulb to an optimum operating temperature with a relatively narrow tolerance band, preferably ±50 K. Since the lamp would quickly devitrify above this temperature range, while the effectiveness of the cycle process would no longer be obtained below the temperature range, the service life is reduced by an excessively high or excessively low temperature. Furthermore, an excessively high lamp temperature can impair units and electronic systems connected to the lamp.
It is known in principle for this purpose to determine the temperature of the bulb and to keep the operating temperature in the above named temperature range by setting a cooling regime.
A number of methods are known for determining the temperature of the bulb.
Firstly, thermocouples can be fastened on the surface of the bulb. However, the usual problems with the use of thermocouples exist in this case. For example, the elements cemented on can become detached from the bulb or supply false values in certain configurations. In addition, such thermocouples require additional outlay on mounting, and electric wiring.
A further possibility consists in determining the lamp voltage and using it with the aid of a known relationship between lamp voltage and temperature for the purpose of determining the latter. However, because the relationship varies according to operating states this method is very inaccurate.
In order to avoid above named disadvantages, it is therefore customary at present to use a pyrometer or infrared camera to measure the temperature of the bulb. However, these methods require a separate optical connection to the bulb, that is to say all the housings surrounding the bulb (for example reflector, lamp housing, projector housing) must have appropriate bores, but this is not always possible in design terms. Moreover, there is the risk of errors owing to vignetting.